1. Field of the Invention
The present invention relates to a polyimide for optical communications, and more particularly, to a polyimide for use in manufacturing optical waveguides, which shows low optical absorption loss at wavelengths for optical communications, and has easily controllable refractive index, good solubility in organic solvent and superior processibility.
2. Description of the Related Art
Rapid advances in information industries have increased the demand for optical materials for use in manufacturing main optical devices associated with future generation high-speed and large-amount information communications business, such as optical power dividers, optical wavelength dividers and the like. At the early stage of research into optical materials, inorganic compounds such as lithium niobate (LiNbO.sub.3) were used as an optical material. However, inorganic compounds such as this inherently have difficulties in preparation and further processing, and thus are not suitable for mass production. For this reason, attention has shifted from inorganic compounds as optical materials to organic optical materials, in particular, polymers, which are more attractive in terms of the cost, processibility and mass production, and this has boosted research into organic optical materials.
However, common polymers absorb light in the near infrared wavelength range of 1,000 to 1,700 nm due to overtone of harmonics by stretching and deformation vibrations of carbon and hydrogen (C--H) bonds. In order to reduce the optical absorption loss, a method for substituting hydrogen of the carbon and hydrogen bonds by fluorine (F) or deuterium (D) has been considered.
However, C--D bonds formed by the substitution of hydrogen by deuterium causes light absorption at a wavelength of 1550 nm, and thus this substitution technique is not appropriate for optical communications materials that utilize light in the near infrared wavelength range of 1,000 to 1,700 nm. Meanwhile, the substitution of hydrogen by fluorine has been verified as yielding an optical material capable of minimizing the optical absorption loss at a wavelength of 1000 to 1700 nm.
Polyimide has been widely known as a semiconductor protective buffering material due to its thermal and mechanical stabilities. Recently, the fluorine substitution technique has been adapted to polyimide having good physical properties to produce low-optical loss optical communications materials.
Optical waveguides consist of a core layer as a light waveguide, and a cladding layer surrounding the core layer, wherein the core layer must have a higher refractive index than the cladding layer. When a fluorine-substituted polyimide is used in the manufacture of optical waveguides, a fluorine-containing monomer and a non-fluorine-containing monomer must be copolymerized in an appropriate mixing ratio to adjust the fluorine content in the resultant polymer, such that the requirement associated with the refractive index of the core and cladding layers is met.
The fluorine content of a polyimide is generally proportional to a decease in refractive index. Accordingly, as the hydrogen of carbon-hydrogen bonds in a polyimide is substituted by fluorine, the refractive index of the polyimide decreases. Thus, use of such a fluorine-substituted polyimide as a material for a core layer narrows the selection range of materials for the cladding layer thereof. Also, in a case where the same polymer is used for both core and cladding layers, the fluorine content of the polymer must be reduced for core layer having a higher refractive index than the cladding layer, which causes optical absorption loss due to the increased carbon-hydrogen bonds. Also, a higher fluorine content in polymer decreases its surface tension, so that use of the fluorinated polyimide as a material for core and cladding layers degrades their adhesion strength and coating properties.